The fabrication of semiconductor devices and/or integrated circuits often requires removing certain materials from a semiconductor wafer while leaving other materials on the wafer. This can be accomplished in a selective etch process that uses an etchant having different etch rates with respect to different materials. To characterize the selective etch process, an etch selectivity is defined as the ratio of the etch rate of one material to the etch rate of another material. For example, an aqueous phosphoric acid solution having a concentration of approximately 85 percent heated to a temperature between 165 degrees Celsius (° C.) and 185° C. is routinely used for removing silicon nitride structures from a semiconductor wafer while leaving exposed silicon dioxide structures on the wafer. At the temperature of 165° C., the phosphoric acid solution etches silicon nitride at a rate of approximately 6 nanometers per minute and etches silicon dioxide at a rate of no more than 0.25 nanometers per minute. The resulting etch selectivity is at least 24:1.
The etch selectivity of an etch process depends on the temperature, concentration, and composition of the etchant. Consequently, the etch selectivity usually changes as more wafers are processed in the etchant. For example, the nitride to oxide etch selectivity of the etch process using the phosphoric acid etchant is approximately 24:1 when the etchant is fresh. After processing approximately 1000 wafers having silicon nitride thereon, the etch selectivity increases dramatically to 50:1 or greater. This selectivity variation adversely affects the efficiency, reliability, and yield of the semiconductor device and/or integrated circuit fabrication processes.
Accordingly, it would be advantageous to have an etch process that has a stable etch selectivity and an apparatus for performing the etch process. It is desirable for the etch process to have a high etch selectivity. It would be of further advantage if the etch apparatus can be adapted from existing etch apparatuses.